Inexorable
(Adj.) cannot be moved or influenced by persuasion or entreaty; unrelenting
Inscrutable
(Adj.) cannot be easily understood; mysterious
Intrepid
(Adj.) without fear; brave
Irascible
(Adj.) easily angered
Jaundiced
(Adj.) prejudiced
Jingoism
(Noun) extreme patriotism; favoring an aggressive, warlike foreign policy
Jocose
(Adj.) joking, humorous
Juggernaut
(Noun) a terrible destructive or irresistible force
Lethargy
(Noun) lack of energy; drowsiness, disinterest
Inert
(Adj.) having no power to act or move; inactive